1. Field of the Invention
The invention relates to an image forming apparatus.
2. Related Background Art
Hitherto, in an image forming apparatus, for example, a color printer, a copying apparatus, a facsimile apparatus, or the like, printing mechanisms of colors of black, yellow, magenta, and cyan are provided. Each printing mechanism constructs an ID (Image Drum) unit and has: an image forming unit for forming a toner image of the relevant color; a copy transfer member for sequentially laying the toner image of the color formed by the image forming unit and transferring onto a recording medium; and the like. In each image forming unit, a toner cartridge is detachably arranged into a main body of the image forming unit and toner of the color is supplied to the image forming unit from a supply port formed in a lower portion of each toner cartridge.
The recording media are picked up and fed one by one from a sheet enclosing cassette. The fed medium is attracted onto a conveying belt by an electrostatic force and conveyed. The toner images of the colors are sequentially laid and transferred onto the medium as mentioned above and a color toner image is formed thereon. The medium is subsequently peeled off from the conveying belt and conveyed to a fixing device. The color toner image is fixed by the fixing device, so that a color image is formed (for example, refer to JP-A-2000-19807).
However, in the above conventional image forming apparatus, when a temperature in the image forming apparatus rises because of an environmental change or an increase in the number of print copies which are continuously printed, that is, the number of continuous print copies, image quality deteriorates.
That is, when the temperature rises remarkably, flowability of the toner in each image forming unit deteriorates and an ability to convey the toner by a developing roller of a developing unit deteriorates. Thus, the toner is continuously agitated in the developing unit and aggregates, reproducibility of a halftone concentration in which a delicate color hue is required deteriorates, gamma characteristic deviates from an ideal characteristic, and smoothness of a continuous gradient change is extinguished.
A charge amount of the toner decreases in environmental conditions of high temperature and high humidity. If an image is formed by using the toner of a small charge amount, the toner is deposited onto a non-image forming area on the recording medium, so that a fog is formed. The toner is softened and slightly coagulated with an increase in temperature. Therefore, when the slightly coagulated toner is deposited onto a charging roller, a photosensitive drum, and the like, an electric potential on the surface of the photosensitive drum decreases and a fog is formed.